This invention is generally directed to liquid developer compositions and, more specifically, the present invention relates to a liquid developer containing certain charge additives. More specifically, the present invention relates to liquid developers comprised of charge enhancing additives of the formulas ##STR2## wherein R.sub.1 is selected from the group consisting of hydrogen and alkyl, and n represents a number and can be 0 (zero), 1, 2, 3, or 4. A preferred charge additive is aluminum-di-tertiary-butyl salicylate or ALOHOS. The developers of the present invention can be selected for a number of known imaging systems, such as xerographic imaging and printing processes, wherein latent images are rendered visible with the liquid developers illustrated herein. Image quality, solid area coverage and resolution characteristics for developed images usually require, for example, sufficient toner particle electrophoretic mobility. The mobility for effective image development is primarily dependent on the imaging system used. The electrophoretic mobility is directly proportional to the charge on the toner particles and inversely proportional to the viscosity of the liquid developer fluid. For example, a 10 to 30 percent change in fluid viscosity caused, for instance, by a 5.degree. to 15.degree. C. decrease in temperature could result in a decrease in image quality, poor or unacceptable image development and undesirable background development, for example, because of a 5 percent to 23 percent decrease in electrophoretic mobility. Insufficient particle charge can also result in poor transfer of the toner to paper or other final substrates. Poor transfer can, for example, result in poor solid area coverage if insufficient toner is transferred to the final substrate and can also cause image defects such as smears and hollowed fine features. To overcome or minimize such problems, the liquid toners of the present invention were arrived at after extensive research, and which toners result in, for example, sufficient particle charge to transfer and maintain their mobility within the required range of the particular imaging system employed. Other advantages associated with the present invention include increasing the desired negative charge on the developer particles and providing a charge adjuvant, or a charge additive, that is superior to other known charge adjuvants like aluminum stearate. The aforementioned desired charge can result in improved image development and enhanced transfer.
A latent electrostatic image can be developed with toner particles dispersed in an insulating nonpolar liquid. These dispersed materials are known as liquid toners or liquid developers. A latent electrostatic image may be generated by providing a photoconductive imaging member or layer with a uniform electrostatic charge, and subsequently discharging the electrostatic charge by exposing it to a modulated beam of radiant energy. Other methods are also known for forming latent electrostatic images such as, for example, providing a carrier with a dielectric surface and transferring a preformed electrostatic charge to the surface. After the latent image has been formed, the image is developed by colored toner particles dispersed in a nonpolar liquid. The image may then be transferred to a receiver sheet. Also known are ionographic imaging systems.
Typical liquid developers can comprise a thermoplastic resin and a dispersant nonpolar liquid. Generally, a suitable colorant, such as a dye or pigment, is also present in the developer. The colored toner particles are dispersed in a nonpolar liquid which generally has a high volume resistivity in excess of 10.sup.9 ohm-centimeters, a low dielectric constant, for example below 3.0, and a high vapor pressure. Generally, the toner particles are less than 30 .mu.m (microns) average by area size as measured with the Malvern 3600E particle sizer.
Since the formation of proper images depends primarily on the difference in the charge between the toner particles in the liquid developer and the latent electrostatic image to be developed, it is desirable to add a charge director compound and charge adjuvants which increase the magnitude of the charge, such as polyhydroxy compounds, amino alcohols, polybutylene succinimide compounds, aromatic hydrocarbons, metallic soaps, and the like to the liquid developer comprising thermoplastic resin, nonpolar liquid and colorant.
U.S. Pat. No. 5,019,477, the disclosure of which is totally incorporated herein by reference, discloses a liquid electrostatic developer comprising a nonpolar liquid, thermoplastic resin particles, and a charge director. The ionic or zwitterionic charge directors illustrated may include both negative charge directors such as lecithin, oil-soluble petroleum sulfonates and alkyl succinimide, and positive charge directors such as cobalt and iron naphthanates. The thermoplastic resin particles can comprise a mixture of (1) a polyethylene homopolymer or a copolymer of (i) polyethylene and (ii) acrylic acid, methacrylic acid or alkyl esters thereof, wherein (ii) comprises 0.1 to 20 weight percent of the copolymer; and (2) a random copolymer (iii) of vinyl toluene and styrene and (iv) butadiene and acrylate. As the copolymer with polyethylene and methacrylic acid or methacrylic acid alkyl esters, NUCREL.RTM. may be selected.
U.S. Pat. No. 5,030,535 discloses a liquid developer composition comprising a liquid vehicle, a charge control additive and toner pigmented particles. The toner particles may contain pigment particles and a resin selected from the group consisting of polyolefins, halogenated polyolefins and mixtures thereof. The liquid developers can be prepared by first dissolving the polymer resin in a liquid vehicle by heating at temperatures of from about 80.degree. C. to about 120.degree. C., adding pigment to the hot polymer solution and attriting the mixture, and then cooling the mixture whereby the polymer becomes insoluble in the liquid vehicle, thus forming an insoluble resin layer around the pigment particles.
U.S. Pat. No. 5,026,621 discloses a toner for electrophotography which comprises as main components a coloring component and a binder resin which is a block copolymer comprising a functional segment (A) of at least one of a fluoroalkylacryl ester block unit or a fluoroalkyl methacryl ester block unit, and a compatible segment (B) of a fluorine-free vinyl or olefin monomer block unit. The functional segment of block copolymer is oriented to the surface of the block polymer and the compatible segment thereof is oriented to be compatible with other resins, and a coloring agent is contained in the toner providing the toner with both liquid-repelling and solvent-soluble properties.
Moreover, in U.S. Pat. No. 4,707,429 there are illustrated, for example, liquid developers with an aluminum stearate charge adjuvant. Liquid developers with charge directors are also illustrated in U.S. Pat. No. 5,045,425. Also, stain elimination in consecutive colored liquid toners is illustrated in U.S. Pat. No. 5,069,995. Further, of interest with respect to liquid developers are U.S. Pat. Nos. 5,034,299; 5,066,821 and 5,028,508, the disclosures of which are totally incorporated herein by reference.
In U.S. Pat. No. 5,223,368 (D/90404), the disclosure of which is totally incorporated herein by reference, there is illustrated a dry toner with the charge additive aluminum-di-tertiary-butyl salicylate
In copending patent application U.S. Ser. No. 986,316 (D/91310), the disclosure of which is totally incorporated herein by reference, there is illustrated a process for forming images which comprises (a) generating an electrostatic latent image; (b) contacting the latent image with a developer comprising a colorant and a substantial amount of a vehicle with a melting point of at least about 25.degree. C., the developer having a melting point of at least about 25.degree. C., wherein contacting occurs while the developer is maintained at a temperature at or above its melting point, the developer having a viscosity of no more than about 500 centipoise and a resistivity of no less than about 10.sup.8 ohm-cm at the temperature maintained while the developer is in contact with the latent image; and (c) cooling the developed image to a temperature below its melting point subsequent to development.
Illustrated in U.S. Pat. No. 5,306,591 (D/92570)is a liquid developer comprised of a liquid component, thermoplastic resin; an ionic or zwitterionic charge director, or directors soluble in a nonpolar liquid; and a charge additive, or charge adjuvant comprised of an imine bisquinone; in U.S. Ser. No. 065,414 (D/92560) a liquid developer comprised of thermoplastic resin particles, and a charge director comprised of an ammonium AB diblock copolymer of the formula ##STR3## wherein X.sup.- is a conjugate base or anion of a strong acid, R is hydrogen or alkyl, R' is alkyl, R" is an alkyl group containing from about 6 to about 20 carbon atoms, and y and x represent the number average degree of polymerization (DP) wherein the ratio of y to x is in the range of from about 10to 2 to about 100to 20; and in U.S. Pat. No. 5,308,731 (D/92571) a liquid developer comprised of a liquid, thermoplastic resin particles, a nonpolar liquid soluble charge director, and a charge adjuvant comprised of a metal hydroxycarboxylic acid. The disclosures of each of the copending patent applications are totally incorporated herein by reference.